Researchers simulated the sinking of marine particle aggregates in oil-dispersant mixtures to assess how Corexit chemical dispersant affects specific biological processes involving marine oil snow formation. The team found that Corexit could significantly enhance or inhibit marine oil snow formation depending on application timing and location and interactions with other water column compounds, making its influence difficult to predict. The scientists are developing a prediction model based on different scenarios and situations to account for these varying factors. The researchers published their findings in Marine Pollution Bulletin: How the dispersant Corexit impacts the formation of sinking marine oil snow.

The decision to use the dispersant Corexit during the Deepwater Horizon incident was and still is controversial and carried ethical components involving first responders and sensitive coastal and deep-sea environments. Study author Uta Passow explained, “The sinking of marine snow to depth and ultimately the seafloor is of central importance for life in the ocean, because sinking particles provide most of the food to the deep, dark, ocean, where no plants can grow. During oil spills, the sedimentation of marine snow is also important, because oil gets trapped within marine snow and is transported to the seafloor, where it may damage deep sea organisms and ecosystems.”

The researchers conducted roller table experiments simulating aggregates sinking through seawater containing phytoplankton cultures, oil, and oil-dispersant mixtures (1:20 dispersant-to-oil ratio, the application ratio used during Deepwater Horizon, or 1:33 ratio). After monitoring the appearance and number of aggregates formed under each treatment, the team tested samples for particulate organic carbon, cell abundance, and transparent exopolymer particles or TEP, the stickily secretion which binds aggregates together.

Many, but not all microbes, increased their TEP production in response to Corexit, suggesting that Corexit might enhance marine snow formation. However, as Passow explained, “It turns out that Corexit not only disperses oil but also disperses TEP, with the end result that marine snow formation is inhibited. The net effect of Corexit addition to spilled oil depends on the relative strength of these two mechanisms.”

TEP dispersion appeared to occur when Corexit molecules had not yet encountered oil. Passow explained, “It appears that once Corexit molecules are associated with oil or natural organic matter, they do not impact TEP. Thus, if the Corexit is already associated with oil when it encounters TEP, there is no effect; if free Corexit molecules encounter TEP, then TEP are dispersed.” Because Corexit increases the amount of dispersed oil in the water column, more oil may be carried to the seafloor despite Corexit decreasing the number of aggregates because the oil load per aggregate is elevated.

Passow noted that because experiments can never really simulate the ocean, results need to be interpreted within the specific question and framework that they simulate. “We feel that in terms of oil and Corexit concentrations our experiments reflect real world conditions after the Deepwater Horizon spill very well, noting that there was presumably a huge range, while we only simulated the more common conditions.”

The authors suggested that responders consider the potential re-enrichment of dispersed oil via marine snow and its transport to the seafloor when making decisions about chemical dispersant application in future spills. “The goal of Corexit addition is the dispersion of oil,” said Passow. “However, the ocean is large and dynamic, and in reality, it is impossible to ensure that every Corexit molecule encounters an oil molecule before encountering other organic matter.”

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.